1. Field of the Invention
The present invention relates generally to ball screws and control rod drive mechanisms and, more particularly, to a ball screw and a fine motion control rod drive mechanism suitable for applications in an advanced boiling water reactor (ABWR).
2. Description of Related Art
A core in which a plurality of fuel assemblies is loaded is disposed, and coolant (light water) to function also as a moderator exists, inside a reactor pressure vessel (RPV) of the ABWR. Control rods (CRs), each being inserted between each pair of adjacent fuel assemblies, are disposed inside the RPV and control of start and stop of the reactor and an output from the reactor in, for example, reactivity compensation and load following is performed by pulling out the CR from the core and inserting the CR in the core. The CR is connected to a fine motion control rod drive mechanism (CRD) disposed at a bottom portion of the RPV and moved vertically by the CRD.
An exemplary CRD is shown in FIG. 3 of JP-2009-174941-A. The CRD includes an outer tube in which a guide tube is disposed. A ball screw and a hollow piston placed on a ball nut of the ball screw and extending upwardly are disposed inside the guide tube. The hollow piston has an upper end portion connected to a CR. A screw shaft of the ball screw is disposed inside the hollow piston. The ball nut engages with the screw shaft via travelling balls. The screw shaft of the ball screw is connected to a rotational shaft of an electric motor and rotated by the electric motor. The ball nut is moved up or down through the rotation of the screw shaft of the ball screw, which, in turn, causes the hollow piston to move up or down. This results in the CR connected to the hollow piston being inserted in, or pulled out from, a core, so that an output from a reactor is controlled.